Cells arrested in the plateau phase in vitro resemble the nondividing GO compartments described in vivo for stem cells and for cells in the viable, but nondividing fractions of some solid tumors. The reentry kinetics of plateau cells in vitro into the cell cycle mimics the recruitment of in vivo GO cells back into the dividing compartment. In addition the in vitro plateau phase survival responses to several cancer chemotherapy drugs have been confirmed with GO cells in vivo, as has the recovery from drug induced potentially lethal damage. For these reasons nondividing or plateau phase cultures in vitro serve as excellent model systems for studies of the effects of cancer chemotherapy drugs on (a) cell lethality; (b) response of the cells to drug induced potentially lethal and sublethal damage; (c) drug sensitization or resistance; and (d) on cell progression kinetics of nondividing cells recruited to reenter the cell division cycle following treatment. The clinical relevance of such studies lies in the fact that the GO cells in vivo are generally not sensitive to cell cycle phase specific drugs. In addition, both dividing and nondividing cells may be able to recover from drug-induced damage. These are the cells which are most likely to contribute to the repopulation or regrowth of the tumor. Therefore, it is extremely important to identify and characterize drugs which will specifically influence cell survival, recovery, sensitization, resistance and recruitment. The systems employed in our proposed studies should produce considerable data that will assist us in understanding these problems, and which will be relevant and valuable in the planning of rational and effective drug protocols for the treatment of solid tumors.